CYA level tanked and can’t get FC


New member
Sep 12, 2021
Daleville, VA
This is a long explanation but hopefully someone can help.

all summer so far, we have maintained good water chemistry (at least we think) until the last couple of weeks and now it is off the rails.

pool is 28000 gallon In-ground liner saltwater pool with an auto cover and this is our 4th season. Our source water is high alkalinity so we do well to keep it below 110.

We take a sample to the pool place regularly. The test results over the past few weeks are below:
FC. TC. PH. TA. CYA. Salt
8/6/21. 0.43. 0.47 7.3. 116. 43. 3200
Added 3# shock
8/21/21. 1.48. 1.5. 7.3. 121. 35. 3300
No recommended changes
9/3/21. 0.09 0.09. 7.2. 125. 35. 3300
Pool was cloudy Added 6# stabilizer to get to 60-80 then added 5# shock. Cloudiness decreased but then returned and has been since 9/9/21
9/10/21 (three tests done that day first 2 at same store a few hours apart and 3rd at different pool place)
1- 0.16. 0.16. 6.7. 148. 20. 3200
2- 0.13. 0.13. 6.7. 154. 5. 3300
Pointed out the result from earlier in the day and the sharp decrease in CYA. Explained I had added 6# of stabilizer on 9/5 when value was 35. They could not come up with any explanation. Suggested I increase the SWG to 45% and test again.
3- 1.4. 1.4. 7. 160. 5. 3400
Explained the situation and they could not explain it and had no recommendations either except to try to get the chlorine up and could add clarifier. Said the only way to decrease CYA is by dilution. We’ve added no fresh water in a month and with auto cover Closed we pump off rain water. There has been no dilution.
on 9/11/21 I added 6# of bagged shock at 1000 in the morning and retested in the afternoon. Zero. Then added 8.4# cal-hypo at 6:30pm. Retested this morning first thing. Zero.

trying to determine where to start. What could have caused the CYA to tank? During this process was told by the pool store we have been going to for 4 years that we do not need any CYA because we have an auto cover. We had not ever been told that and in fact water chemistry stays consistent when between 50-70 so we have been trying to keep it there.

I have a troubleshooting request in other Auto Pilot technicians for tomorrow. I can’t detect chlorine coming from the jets when on boost mode which is unusual. But even if SWG has failed, that wouldn’t cause a decrease in CYA level like that, right?

some research revealed maybe ammonia/nitrates? Recommend liquid bleach 10% or 12.5% to get to FC of 10ppm and then holding above 5 ppm for 30 minutes.

are there pool water chemistry experts available to whom I could send a sample and then consult? The pool stores have no idea what is happening.

Any suggestions would be appreciated.


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Welcome to the forum!
You need a proper test kit. I suggest the TF-100 or Taylor K2006C. A proper test kit is needed to get the accurate water chemistry results needed to follow the TFP protocols.
Once you get your test kit, post up your test results. We do not provide guidance based on pool store tests.
And you are correct, your CYA cannot just disappear.
I suggest you read ABC's of Pool Water Chemistry.
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